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Modern experimental research theory includes plans for statistical multifactorial analysis. The advantage of this theory is that all factors are changed simultaneously so that a minimum number of experimental points distributed in the experimental hyperspace is obtained, which results in much lower costs and shorter time of experimental tests. The process of decontamination of polychlorinated biphenyls (PCB) from transformer oils is based on a dechlorination reaction using a reagent mixture (KPEG) consisting of potassium hydroxide (KOH) and polyethylene glycol (PEG), whereby the PCB molecule is chemically decomposed. Using 23 orthogonal plans, the influence of the mass ratio of KPEG/oil, KOH/PEG molar ratio and temperature on the dechlorination reaction were investigated to optimize this process. The success of the reaction was evaluated by determining the efficiency of chemical degradation of PCBs (DRE) from transformer oil. The DRE values were from 81.81% to 99.81%. Using dispersion analysis, the significance and degree of interactivity of the examined factors on DRE were determined, and using regression analysis, a multilinear regression model was set up.

The project funded by the Science Fund of the Republic of Serbia, Development of Green Technology to Mitigate Power Transformer Failures Induced by Elemental Sulphur and Change Current Hazardous Practice in Transformer Oil Regeneration, under the acronym GreenCleanS focuses on developing a new technology for removing elemental sulphur molecule (S₈) from mineral insulating oils used in power transformers (PT) utilization an innovative low-temperature desulfurization process. The technology development includes experimental work at the laboratory scale and pilot scale level (technology readiness level, TRL 4), as well as the demonstration of the technology in an operational environment, involving the treatment of 3 tons of oil in a mobile desulfurization unit to achieve TRL 7. The implementation of this innovative technology will contribute to reducing the number of transformer failures caused by mineral oils containing S₈, a highly corrosive compound.

The Republic of Serbia signed and ratified the Stockholm Convention on Persistent Organic Pollutants. Law on ratification of the Stockholm Convention was adopted and actions were defined through the National Implementation Plan and the Action Plan. On their basis Project Proper Management and Final Disposal of Polychlorinated Biphenyls (PCBs) was created. Institute Nikola Tesla (INT) participated in the realization of the project. In the period from 2015 to 2022, 601 tons of PCB oil was successfully decontaminated by the INT mobile plant, i.e. the PCB content was below 50 mg/kg, which classified the power transformers (PT) as non-contaminated [1,2]. If PT becomes waste or in case of necessary repair or oil regeneration, the required limit value is 10 mg/kg. According to data from INT database there are still 2637 PTs containing PCBs in the concentration range of 2 to 50 mg/kg. In this paper application of the environmentally closed INT decontamination technology for achieving low values of PCB in the oil is shown.

Most power transformer failures are associated with the presence of corrosive sulfur compounds in mineral insulating oil, of which dibenzyl disulfide (DBDS) is the best known and most widespread corrosive sulfur compound. The oil of failed transformers usually contained DBDS, a compound with a pronounced affinity for the formation of copper (I) sulfide (Cu2S). Copper (I) sulfide is a compound that does not dissolve in oil, conducts electricity, and can be deposited on the paper insulation or copper conductor of the transformer. Created conductive paths between the windings can initiate turn-to-turn breakdown and failure of the transformer. Several technological procedures have been developed as a solution to oil corrosivity. The application of desulfurization technology developed by the Electrical Engineering Institute "Nikola Tesla" enables the efficient removal of DBDS and aging products from mineral insulation oils.

The reliability of power transformers operation largely depends on the insulation system condition, especially on the condition of paper insulation. Determining the condition of paper insulation of transformers in use cannot be made by direct measurements. The most common way of assessing the paper insulation condition is by analyzing the content of 2-furfural dissolved in the insulating oil and by applying correlations between the degree of paper polymerization and the 2-furfural content in the insulating oil. The concentration of 2-furfural in the oil depends on the moisture content of the insulation system and the oxygen and acid content in the oil. In order to gain a better insight into the cumulative content of 2-furfural produced by paper degradation, excluding changes in the 2-furfural content caused by migration between paper and oil at different operating temperatures, dependence of the degree of polymerization of kraft paper to 2-FAL content in the oil, corrected to a reference temperature of 20°C, for a moisture content in cellulose of 1-1.5% and 3-4% is presented in this paper. This diagram has been verified on 4 transformers according to available data from the database of the Electrical Engineering Institute Nikola Tesla. In the future, this diagram should be completed for moisture content of 2.5% in cellulose insulation.

The exploitation of PCB contaminated power transformers and those which contain corrosive sulphur is related to the greatest risks to the environment, due to possible environmental accidents and faults and failures of transformers. The equipment contaminated with PCB require special attention, more frequent testing of transformer condition and control of oil leaking, while for corrosive oils standards and requirements defined in CIGRE TB 378, CIGRE TB 625, IEC 60296 and IEC 60422 are restrictive for the use of corrosive oils. Overview of the problems with PCB contaminated equipment and corrosive sulphur is provided in accordance to the latest knowledge in these areas. PCB decontamination, desulfurization and oil regeneration process is described by the application of patented technology developed in the 'Nikola Tesla' Institute. Summarized results of decontamination process of 310 PCB contaminated power transformers with different voltage levels and power and removal of corrosive sulphur from 29 transformers show the efficiency of this process for simultaneous removal of PCB, corrosive sulphur and oil ageing products.